The present invention relates to an electronic motor drive circuit for a brushless DC motor. In particular, the present invention is directed to an electronic motor drive circuit in which current sensing efficiency is improved by eliminating the use of a current sense resistor to sense motor current.
Efforts have been made to provide an electronic drive circuit for a brushless DC motor having a reduced power dissipation as compared to linear operation, by operating a set of pass transistors in a saturated switching mode and utilizing a "flyback" effect to sustain a relatively constant motor current. An example of such a drive circuit is illustrated in FIG. 1. A set of motor windings W.sub.1, W.sub.2 and W.sub.3 are interconnected between a first set of FET transistors P1, P2, and P3, and a second set of FET transistors N1, N2 and N3 in a bridge circuit. The transistors are operated as saturated switches, thereby dropping the operating voltage principally across the motor inductance rather than across the transistors or winding resistance.
The commutation logic 10 establishes a current path through a selected first transistor (P1, P2 or P3), a pair of windings (W1, W2 and/or W3), and a selected second transistor (N1, N2 or N3). For example, an established current path through transistor P1, windings Q1 and Q2, and transistor N2 is shown in FIG. 2. A current sensing resistor 12 is also provided to sense the motor current. A comparator 14 triggers a one-shot circuit 16 when the voltage across the resistor 12 reaches a predetermined threshold voltage. The output signal from the one-shot 16 is provided to the commutation circuit 10, which then switches off the transistor P1 thereby blocking current flow through the transistor.
The load voltage across the motor inductance then reverses polarity as the field begins to collapse, thus reversing the polarity of the voltage across the transistor N2. A current path is then established through the body diode of another of the second transistors, for example N1, and the load current that was flowing through the transistor P1 thereafter flows through the series-connected second transistor N1 (alternatively, transistor N1 can be switched on to provide a current path). FIG. 3 illustrates the current path through the second transistor N1 after the transistor P1 has been switched off. As the load voltage has been reversed, the load current decreases until the one-shot 16 reverts to its original state. Thus, the load current through the motor windings is stabilized within predetermined limits while power dissipation is reduced by operating the transistors as saturated switches.
The above described motor control circuit fails to achieve maximum efficiency, however, due to the power loss across the sense resistor 12. Thus, it would be desirable to develop a motor driver circuit wherein the current sense resistor is eliminated to improve the efficiency of the circuit.